1. Field of the Invention
The present invention relates to an apparatus for heating ceramics at high temperatures and at a controlled rate by means of microwaves.
2. Description of the Prior Art
Speciality ceramics which are used as structural materials withstanding high temperatures exhibit excellent properties, including heat resistance, anticorrosion, and abrasion resistance. They find extensive application in automobiles, aircrafts, electronic materials, etc. In order to improve the quality, there is a growing tendency toward higher purification and higher density of ceramics. As a result, it has become increasingly difficult to sinter and shape ceramics, which constitutes an impediment to extension of application of ceramics.
In recent years, microwave heating has been proposed to sinter or shape these ceramics. A well known application of microwave heating is domestic microwave oven. Also, microwave heating finds industrial applications, such as vulcanization of rubber, drying of wood and printed matter, and drying and sterilization of food. These materials are easy to heat by means of microwaves, because they have large dielectric loss factors given by .epsilon..sub.r tan .delta.. Generally, however, ceramics have small dielectric loss factors and so they are difficult to heat by means of microwave energy.
In an attempt to effectively heat ceramics, a method using a cavity resonator has been proposed. Specifically, a mass of ceramic is inserted in the resonator. Microwave power is caused to enter it so that the resonator may resonate. Thus, the mass is heated. Those which have been heretofore reported to be heated by this method are generally ceramics having dielectric loss factors greater than 1 and ceramics of low purities less than 50%. It has been difficult to heat ceramics having high purities and dielectric loss factors less than 0.1 to high temperatures by this method.
Also, attempts have been made to match a cavity resonator, using an EH tuner or stub tuner. However, it has been impossible to heat ceramics which exhibit small dielectric loss factors at ordinary temperatures up to high temperatures for the following reason. When these ceramics are heated, their dielectric loss factors change rapidly, greatly increasing the power of microwaves reflected from the cavity resonator.
An improved method of heating using a cavity resonator consists in driving a plunger in the resonator. The resonant frequency of the resonator is adjusted by the movement of the plunger, in order to improve the efficiency of heating of ceramic. However, as a ceramic is heated in this way, the reflected power increases rapidly, making it impossible to heat it to high temperatures.
Microwave heating has the advantage that it can heat materials rapidly. However, it is very difficult to control the heating velocity. One conventional method of controlling the heating velocity is to control the power of microwaves and the time for which the microwave is applied. Another conventional method consists in adjusting the power of microwaves according to the heating temperature. Where ceramics whose dielectric loss factors depend strongly on temperature are heated by either method, the dielectric loss factor changes sharply with temperature. Therefore, it has been difficult to regulate the power against temperature variations. Hence, accurate control of temperature has been impossible. Especially, when a ceramic is heated rapidly to a high temperature, a large temperature error results. This means that the material is frequently heated above the intended temperature. As a result, nonuniform heating, or deterioration of characteristics in the material takes place, thus greatly lowering the reliability of the heating.